This project undertakes a systematic observation of the epithelium of the urinary bladder through routine light microscopy and electron microscopy. Specimens are obtained from symptomatic patients via cystoscopy or operative procedures such as partial and complete cystectomy. The object of the investigation is to re-define with the help of electron microscopy such categories as normal, metaplastic, inflammed, atypical and neoplastic epithelia. It is hoped to identify the earliest morphologic expression of malignancy and to construct the evolution of overt cancer from this precursor or incipient state. Preliminary results of the electron microscopic survey indicate a lack of understanding of the normal state of the epithelium, and this must be rectified. Additionally, various preoperative therapeutic measures and operative procedures provide confusion. In spite of these problems, the procedures are feasible and, with control, reproducible. They serve to indicate a similarity between the ultrastructural features of severe atypia of the transitional epithelium (carcinoma in situ), papillary carcinoma, and solid (transitional) carcinoma. These differ from epidermoid (squamous) carcinoma which in turn may be distinguished from squamous metaplasia.